The tower being destroyed was a sensor tower, not a shield tower. The shields on the Executor were taken down by a concentrated bombardment from a significant percentage of the Rebel fleet. Ackbar didn't say 'Concentrate all fire that Super Star Destroyer' because he was for the lulz.' If all it took was two guided missiles to destroy the entire ship, they would do that.

Couldn't happen, unless there is some other KOTOR era super-ship that isn't public knowledge. The Executor is itself a super weapon, IMO. It would be foolhardy to attack it with anything less than another super weapon or a fleet of capital ships.